1. Technical Field
This invention relates to methods and compositions for the construction of anodes, cathodes and batteries of the lithium ion secondary (rechargeable) type. In particular, the invention relates to fullerene-based secondary cell components and methods and compositions for their construction.
2. Description of the Background Art
Lithium ion secondary (rechargeable) cells are commonly used as power sources in portable electronic devices. Such rechargeable cells generally use a lithium transition metal oxide (very often Li2CoO2 (lithium colbaltate)) cathode and an anode composed of a highly porous carbonaceous material, usually graphite or a pyrolyzed organic material. A lithium ion-soluble electrolyte is placed between the two electrodes, and the cell is charged. During the electrochemical process of charging, some of the lithium ions in the cathode material migrate from the cathode to the carbonaceous anode layer and completely intercalate into it. During discharge, the negative charge held by the anode is conducted out of the battery through its negative terminal, and the Li+ ions migrate through the electrolyte to their original location in the cathode. When this migration is complete, the cell has been completely discharged, and the lithium ions in the battery are at an electronic “ground state.”
The porous carbonaceous anode material can reversibly incorporate ions within its crystal lattice with only small structural changes. Structurally, graphite is a planar sheet of carbon atoms arranged in a honeycomb. The carbon layers are stacked to form what is commonly known as hexagonal (2H) or rhombohedral (3R) graphite. A certain amount of random stacking or disorder in the structure is common in graphite and in other carbon forms such as cokes, petroleum cokes, synthetic graphites, carbon blacks and the like.
Lithium ion batteries possess four main advantages over other rechargeable cells such as nickel metal hydride, nickel-cadmium, and lead-acid cells. First of all, lithium ions, due to their small size, can intercalate between carbon layers more easily and completely than larger battery ions such as nickel and lead. Because of this property, lithium ion cells do not form battery “memory” ion channels. Secondly, the electrical potential of lithium ions is the most similar to graphite (carbon) of any metal ion. This allows the easiest possible charge transfer between carbon battery anodes and migrating lithium ions. Better charge transfer gives these cells more efficient, complete, and long-lasting discharge rates. Thirdly, lithium cells are much less toxic than comparable secondary cells which use lead, cadmium or nickel metal ions. Fourthly, lithium, the lightest metal, has a high charge-to-mass ratio and thus produces a battery of lighter weight.
Although current lithium battery technology constitutes an enormous advance over that of previous commercial secondary cells, there are a number of shortcomings involving the current materials used as battery electrodes. Commercial lithium secondary cell cathodes are composed of lithium salts, such as lithium colbaltate, which allow less than 50% lithium ion migration to the anode. Largely due to the presence of two distinct ionization energies in the active cathode material (i.e., [Li2CoO4]⇄[Li+][LiCoO4]⇄[2Li+][CoO42−]) the battery never achieves its full theoretical charge potential. To overcome the activation energy barrier for lithium ion formation in the cathode, more energy must be put into the battery during charging than is returned during discharge. Some of the energy used to overcome this barrier is regained when the original compound is reformed, but some is lost as heat. Cathodes which do not have this high energy barrier would require less energy to charge and would achieve a more complete charge. Therefore, materials with more efficient charge transfer chemistries would be highly desirable.
The anodes used in lithium ion batteries also have certain characteristics which prevent optimal performance. Due to the nature of graphitic and pyrolyzed carbon anode materials, electrodes made from them are inevitably irregularly sized, non-directionally specific, and possess non-predictable “pockets” of charging where lithium ions can intercalate into the anode. The extended structure of the carbon compound chosen for the anode therefore influences both the total amount of lithium which can be intercalated within it and at what voltage. The electrical charge stored in the anode must be able to freely migrate between all points of the anode and the negative cell terminal for optimal performance. Poor orientational control increases resistance in the anode and reduces cell charge transfer efficiency as well. The carbon electrodes currently in use commercially employ various forms of amorphous or graphene layered carbon. By their very nature, the structure of these materials cannot be controlled at a molecular level to maximize their affinity for lithium ions.
Heat-treating these prior art carbonaceous materials increases their crystallinity and affects both their structure and their ability to intercalate lithium. Current methods of hydraulic compression and heat treatment significantly help this problem by creating a more regular structure, but the problem cannot be completely solved until the anode is manufactured in a manner such that the order and porosity is more controlled. Therefore, new carbon materials which can be manufactured with more controlled structure would be highly desirable and produce more efficient batteries.
Fullerenes are spherical or partially spherical aromatic compounds composed solely of triconjugate (Sp2-hybridized) carbon atoms. As such, they resemble an ideal graphite sheet, but for the strain which their spherical shape imposes on the normally planar aromatic structure. This strain causes fullerenes to be more reactive than a continuous aromatic sheet. Fullerene molecules are highly electronegative as well, and possess unusual magnetic and electrical properties.
Improvements in the art of lithium ion batteries and battery electrodes therefore would be highly desirable. Such electrodes would enable the manufacture of smaller, lighter rechargeable batteries with longer life and more efficient charging for use in portable electronic devices such as telephones, CD players, hearing aids, computers, and the like, or any device where a high efficiency light weight rechargeable battery is desirable.